This invention relates to the manufacture of printed-circuit boards and more particularly to an adaptation of such boards for efficient electroplating of certain portions of the metallic wiring deposited thereon such as the contact finger areas.
Electrical circuit boards having in one or another manner wiring printed thereon have long been known in the art and have contributed substantially to reducing the cost of fabricating and maintaining complex electrical systems as well as to reducing the size of such systems. A printed-circuit board is readily adapted to large-quantity production.
Although the advantages of printed wiring circuitry are obvious, stringent requirements for long term reliability of the board may present particular problems in their fabrication. One area of particular concern is in the preparation of those portions of the printed wiring termed the contact finger areas or contact tabs. An array of one or more of these tabs comprise, in effect, a plug intended to be mated with a female connector assembly having spring-loaded pressure contacts which establish electrical continuity between the circuits and circuit components mounted on the board and other parts of the electrical system external to the printed-circuit board. For long-term reliability of the board and the system as a whole, particularly in digital computer applications where signal currents may be very small, it is essential that this electrical continuity not deteriorate either due to surface corrosion or other physical or chemical change in the layers of metal comprising the contact tabs. To achieve this end, the contact tabs, normally comprising a base layer of copper, and sometimes an additional layer of nickel, are commonly given a final exterior coating of a precious metal such as pure or alloyed gold. The preferred method of producing the precious metal coating is by electroplating. It is desirable to use only the minimally sufficient amount of such precious metal in the electroplating process due, obviously, to its high cost.
Printed-circuit boards are fabricated by multi-step processes well known to those skilled in the art. In the base of all circuit boards is at least one flat sheet or plate of insulating substrate usually made of molded epoxy-glass or phenolic resin material. The blank base for the board is commonly rectangular in shape. The dimensions of the blank board need not be precisely controlled since the finished portion of the printed-circuit board is ordinarily cut away from the remainder of the substrate by precise blanking or routing procedures. The blank board is ordinarily made large enough to accommodate certain expendable auxiliary printed wiring which is provided to be useful in certain intermediate steps of the processing. However, a significant portion of this auxiliary wiring is itself plated with precious metal. As much as fifty percent of the precious metal plated on the board may be deposited on auxiliary wiring. Eventually, when the finished portion of the printed-circuit board is cut away from the remainder, the portion of the substrate having the auxiliary wiring thereon is left as scrap. While the precious metal plated on the scrap may be reclaimed, the reclamation process itself is an added cost in printed-circuit board production.
It has been the prior art practice to include a printed-wire bus bar connected to each of the contact tabs as part of the auxiliary wiring on a substrate. This bus bar enables the precious metal electroplating of the contact tabs by completing a path for electroplating current from each of the contact tabs to a source of negative potential. However, in the prior art practice, not only are the contact tabs subject to contact with electroplating baths but so also is the bus bar and each bus bar connection to a contact tab. The present invention makes the use of such a printed-wire bus bar unnecessary by providing a contact tab electroplating current path which is protected from contact with electroplating baths.